The invention relates to cutting structure for rotary drill bits for use in drilling or coring deep holes in subsurface formations.
In particular, the invention is applicable to cutting structures for rotary drill bits of the kind comprising a bit body having a shank for connection to the drill string and an inner channel for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting structures. Each cutting structure comprises a cutting element, often in the form of a circular disc, having a hard cutting face formed of polycrystalline diamond or other superhard material and mounted on a carrier which is, in turn, mounted on the bit body.
Conventionally, each cutting element has usually been a preform comprising two layers: a hard facing layer formed of polycrystalline diamond or other superhard material, and a backing layer formed of less hard material, usually cemented tungsten carbide, the two layers being bonded together during formation of the cutting element in a high pressure forming press.
In one common form of drill bit of the above-mentioned type, the bit body is machined from steel and each cutting structure includes a stud or post to which the polycrystalline diamond preform is brazed, the stud or post being received and secured, for example by shrink fitting, in a socket in the steel bodied bit. The post or stud is formed from a hard and erosion resistant material such as cemented tungsten carbide.
Conventional two-layer preforms of the kind referred to above are only thermally stable up to a temperature of about 700.degree. to 750.degree. C. Due to this limitation, problems have arisen in brazing the preforms sufficiently securely to the stud or post. Generally speaking, the strength of a brazed joint depends on the liquidus temperature of the braze alloy--the higher the liquidus temperature the greater the strength. Accordingly, if the brazing is carried out at a temperature which the preform can withstand the resultant brazed joint may not be sufficiently strong to resist the substantial mechanical forces to which it is subjected during drilling. The joint may also fail as a result of high temperatures reached during drilling.
If a higher temperature brazing process is employed, however, sophisticated cooling techniques must be employed to protect the two-layer preform from the high temperature at which brazing takes place. Such techniques are described, for example, in U.S. Pat. No. 4,319,707.
There are, however, now available polycrystalline diamond materials which are thermally stable at higher temperatures, for example temperatures over about 1100.degree. C. Such a thermally stable diamond material is supplied by the General Electric Company under the trademark `GEOSET`.
This material has most commonly been applied to rotary drill bits by setting pieces of the material in the surface of the bit body so as to project partly from the surface, using similar methods to that previously used to mount natural diamonds in a bit body. However, since such thermally stable elements do not have a backing layer to provide support, they have normally been of substantially greater thickness, in the cutting direction, than the diamond layer of conventional two-layer preforms in order to provide the necessary strength.
Thermally stable polycrystalline diamond cutting elements are available which are of similar shape to the conventional two-layer preforms, for example in the form of thin circular discs, and various methods have been devised for mounting such cutting elements on a bit body. The use of elements of such shapes is advantageous since they provide a degree of self-sharpening. This is due to the fact that the material on which the cutting element is mounted will be less hard than the polycrystalline diamond material of the cutting element, and thus wears away more rapidly in use.
The use of thermally stable cutting elements has the advantage, over conventional two-layer preforms. that higher brazing temperatures may be used to obtain the required strength for the brazed joint, without the need for sophisticated cooling techniques and yet without the risk of the preform being damaged by overheating. The use of such thermally stable preforms also has the advantage that they are less liable to damage through overheating during use on a drill bit. The preforms may also be cheaper to manufacture than two-layer preforms, one possible reason for this being that since they are not initially formed with a backing layer they may be thinner than two-layer preforms so that more of them may be formed in the press at the same time.
The use of thermally stable cutting elements may also be advantageous in the case where the bit body is formed by a powder metallurgy process. In such process a mould is packed with powdered material, such as tungsten carbide, which is then infiltrated with a metal alloy binder, such as copper alloy, in a furnace so as to form a hard matrix. The maximum furnace temperature required to form the matrix may be of the order of 1050.degree. to 1170.degree. C. As previously mentioned, conventional two-layer preforms are only thermally stable to a temperature of about 700.degree. to 750.degree. C. and for this reason it has been necessary to mount the cutting elements on the bit body after it has been formed in the furnace. However, if thermally stable preforms are used the preforms may be located in the mould so that they become embedded in the surface of the bit body at the same time as the infiltrated matrix is formed in the furnace. Again, however, in order to obtain a degree of self-sharpening it is desirable to be able to use thermally stable polycrystalline diamond cutting elements which are of similar shapes to the conventional two-layer preforms, for example in the form of thin discs.
Thus, in order to facilitate the mounting of a thermally stable cutting element on a bit body, whether it be of steel or of infiltrated matrix, it would be advantageous to incorporate the cutting element in a cutting structure including a carrier to which the thermally stable cutting element is brazed. Hitherto, it has been considered that such carrier should be formed of tungsten carbide, which, as previously mentioned, is the material normally used for the carrier in the case of conventional two-layer, non-thermally stable preforms. However, problems arise in brazing thermally stable cutting elements to carriers of tungsten carbide. In particular, the substantial difference in coefficient of thermal expansion between the polycrystalline diamond material and tungsten carbide, at the brazing temperature, results in substantial stresses arising when the cutting structure is cooled after brazing, resulting in failure of the cutting element and/or carrier or, at the very least weakening of the bond. This disadvantage offsets the advantage of being able to use a higher brazing temperature. The present invention sets out to provide a method of manufacturing a cutting structure, using a thermally stable polycrystalline diamond cutting element, in which at least certain of the problems encountered hitherto may be overcome.